In 1995, Seyfried began her acting career at age nine. In her hometown Allentown, Pennsylvania, she attended classes at Civic Theatre of Allentown. She signed with Image International Agency (IIA) in Allentown, before switching over to work at Bethlehem's Pro Model Agency and eventually signing with Wilhelmina Agency in New York City, New York. Seyfried's modeling career ended when she was seventeen. During her modeling career she appeared in print ads for clothing companies; including Limited Too with Leighton Meester.



While modeling, Seyfried took voice lessons from eleven to seventeen. She trained with a Broadway voice coach for nearly five years and two years in studying opera in which it led her to appear in the New York stage productions, the revival of Grease and A Christmas Carol. From there she began acting as an extra in the daytime drama television series Guiding Light; her role on the show was uncredited. In 2000, she portrayed Lucinda Marie "Lucy" Montgomery #2 on the television show As the World Turns. Appearing on the show as a recurring character, her role on the show lasted until 2001. After appearing in 27 episodes, Seyfried left the show as a result of creative conflicts. From 2002 to 2003, Seyfried played the recurring role of Joni Stafford on the American Broadcasting Company (ABC) broadcasted show, All My Children. In 2003, Seyfried auditioned to play the mean-girl role of Regina George in the Mean Girls starring Lindsay Lohan; the role eventually went to Rachel McAdams, but the producers of the film selected Seyfried to play one of Regina's two dim-witted "Plastic" friends and sidekicks, Karen. The film was a box-office success, earning over $129 million in its theatrical run. Seyfried's performance in the film earned her, along with Lohan and McAdams, an MTV Movie Award in the category of "Best On-Screen Team."



Seyfried auditioned to play the title character on UPN's television series Veronica Mars.[5] Although the role eventually went to Kristen Bell, Seyfried portrayed the title character's murdered best friend, Lilly Kane. Although her character was only shown in flashbacks, Lily was vital part of the first season's plot and made Seyfried one of the show's familiar faces. Seyfried appeared in ten episodes from 2004–2005. In 2005, Seyfried played the lead character, Samantha. A role written specifically for her, in one of the nine parts in the film Nine Lives, comprising nine short films with different themes and an ensemble cast. For her performance in the film, Seyfried won an award from the Locarno International Film Festival, for Best Actress, which she shared with the film's other female leads. The same year she played supporting character Mouse in the critically acclaimed independent film, American Gun. In 2006 Seyfried appeared in five episodes on Wildfire as Rebecca and had lead role as Chrissy in the award winning short film titled Gypsies, Tramps & Thieves, by writer-director Andrea Janakas. Seyfried also contributed a minor role as Julie Beckley in Alpha Dog. From 2004 to 2006, Seyfried made multiple guest appearances on television series, including House, M.D., Justice, Law & Order: Special Victims Unit and CSI: Crime Scene Investigation. In 2008, Seyfried also appeared and played a supporting role in the independent short film, Official Selection.